This invention relates to ventricular shunt systems, and more particularly to a dual pressure shunt valve assembly which may be operated to maintain either one of two cerebro spinal fluid pressures in the ventricles of a patient's brain.
The treatment of hydrocephalus frequently involves the implantation of a ventricular shunt for draining excess cerebral spinal fluid (CSF) from the ventricle in the brain. The shunt generally consists of a cerebral catheter inserted through the brain tissue into the ventricle and connected through a one-way valve system to drain into the jugular vein or other resevoir in the body. The shunt provides for removal of excess CSF from the ventricle and consequent reduction in its size. Control over the drainage is provided by the one-way valve, which is normally biased to operate at a fixed back pressure at normal flow.
Although hydrocephalus is frequently associated with an abnormally high CSF pressure, there are numerous cases where hydrocephalus is associated with the CSF at normal pressure, see Ojemann, Robert G., Normal Pressure Hydrocephalus, Clinical Neurosurgery, Vol. 18, pp. 337-370, 1971.
An analysis of the hydromechanics involved in normal pressure hydrocephalus syndrome leads to the conclusion that the effective expansion force on the ventricles is not dependent on the CSF pressure alone, but is the product of the CSF pressure and ventricular area. In other words, in the presence of ventricular enlargement the pressure may be "normal." Thus, in normal pressure hydrocephalus the ventricle remains enlarged, because the area subjected to the pressure of the CSF is larger than normal and hence the total force on the brain tissue, the product of the pressure times the area, is excessive. (See, S. Hakim and R. D. Adams., The special clinical problem of symptomatic hydrocephalus with normal cerebrospinal fluid pressure: Observations on cerebrospinal fluid hydrodynamics. J. Neural. Sci., Vol. 2 pp. 307-327, 1965).
In addition to the forces developed by the CSF pressure, the brain tissue is subjected to a counterforce developed by the venous pressure within the intraparenchymatous system, within the brain tissue itself. Whereas CSF pressure tends to enlarge the ventricles, the venous pressure tends to reduce their size. These two forces are normally in balance so that the ventricular size relationship does not increase nor decrease, but remains constant through life.
Accordingly, the aim in treating hydrocephalus by shunting procedures in not merely to arrest the condition, but to restore, as much as possible, normal ventricular size. Once hydrocephalus has developed, this restoration is accomplished by reversing the imbalance of the two forces acting on the brain parenchyma. The CSF pressure must be reduced to offset the increased force developed by virtue of the enlarged area of the ventricles. Then the forces developed within the venous system can cause the compressed brain tissue to "spring back" against the lower CSF force, and the venous bed will regain its lost volume and free flow. This way, the brain metabolism becomes normal and the tissue recuperates.
In treating hydrocephalus, a reduced CSF pressure is established by a shunt which includes a oneway valve having an operating pressure equal to the desired CSF pressure. With the shunt in place the CSF pressure remains at a maximum level determined by the implanted valve, and drainage of CSF from the ventricles continues as long as CSF pressure is not less than the operating pressure of the valve. Since the correction of normal pressure hydrocephalus requires the implantation of a valve having a lower than normal operating pressure, e.g. approximately 30-40 mm H.sub.2 O, depending on the size of the ventricles. The CSF pressure remains lower than normal as the ventricles decrease in size. On the other hand, since the venous pressure remains at normal levels, the force imbalance is reversed. The venous system force becomes greater than the CSF system force because of the progressively smaller ventricular area and lowered CSF pressure. Accordingly, once the ventricle is again normal size, the intraventricular CSF pressure must be broughtback to normal levels. Otherwise, there is not enough force within the ventricles to keep them normally expanded.
If lower than normal CSF pressure is maintained, overcorrection of hydrocephalus may cause undesirable pathological consequences, such as cerebral edema, "slit ventricles" and microcephaly. In other cases, complications such as subdural hygromas, hematomas, and overlapping of the skull bones are known to occur.
In brief, the problem is corrected by maintaining the intraventricular CSF at a reduced pressure until such time that the ventricular volume is reduced to normal size. At this point, the valve provided for the initial drainage should be replaced with one having a closing pressure equivalent to a normal CSF pressure, 125-150 mm H.sub.2 O, or higher to act only as a safety valve if the bodies own absorptive system is functioning partially or totally.
A second reason for exchanging the initially installed reduced pressure valve for a high pressure valve at a later time stems from the theory that a high pressure valve may tend to coax the body's normal drainage systems to function, hence preventing the patient from becoming valve dependent.
A third reason to exchange pressures is the theory that, during the initial healing stage, a higher pressure should be maintained to prevent the possibility of hematomas or hygromas developing at the intrusion site and to lower the pressure a few days or hours later to reduce the ventricle size.
Accordingly, it is an object of the present invention to provide a dual pressure valve assembly for use in series with a ventricular catheter and an implantable drain system such as those described, which can be easily manipulated so as to change from a low pressure mode to a higher, closer-to-normal, pressure mode and thereby eliminates the necessity of replacing the shunt system. Preferably, the adjustment should be accomplished quickly, easily, and effectively by a simple manual procedure performed without surgery. Other objects of the invention are to provide a ventricular shunt system which is dependable, simple, inexpensive to manufacture, useful in treating both high and normal CSF pressure hydrocephalus, and capable of being shifted as necessary, at various times in the hydrocephalus treatment period, in a rapid and safe manner, the position of the shiftable valve being easily detected by feel or normal x-ray techniques.